The microscopic magnetic-induction field ''seen by each nuwork which relations can be safely extended to the present cleus'' in a material medium and which is generated by a rapidly NMR context in which the nuclear spin magnetization is time-dependent spin magnetization gives rise to surprising new manipulated during the experiment, and hence becomes rapfeatures in high-resolution nuclear magnetic resonance experiidly time dependent and unrelated to equilibrium properties. ments. The purpose of the present paper is to show how the rela-Another source of complications is the introduction of the tions between the macroscopic average fields, the magnetization, notion of ''secular part'' of the microscopic field, often very and the microscopic fields (which were studied and clarified long early in the discussions given in the dynamical NMR context.
ago at thermal equilibrium) can be extended to the present NMR
The present paper is an attempt to discuss the microscopic context in which the magnetization can become rapidly time defields B m Γ
m 0 H m seen by the nuclei in a way which corrependent and unrelated to thermal equilibrium properties. α§ 1996 sponds to the present needs of NMR: the nuclear magnetiza-Academic Press, Inc.
tion is considered as an autonomous, time-dependent quantity (not an equilibrium property), and the separation between the secular and ''nonsecular'' parts of B m is discussed 234